The Unique Dioxygen Activation Mechanism of Human Indoleamine 2,3-Dioxygenase

Abstract

Human indoleamine 2,3-dioxygenase (hIDO) is an intracellular heme-containing enzyme, which catalyzes the initial and rate-determining step of L-tryptophan (L-Trp) metabolism via the kynurenine pathway in nonhepatic tissues. We have employed stopped-flow methods to study the L-Trp and oxygen binding kinetics of hIDO and the associated oxygen chemistry at pH 7.4 and 20 oC. We found that the binding rate constants of L-Trp to ferric and ferrous hIDO are 5.5∗10(3) and 1.2∗10(5) M-1s-1, respectively. In contrast to other dioxygenases or monooxygenases studied to date, under physiological conditions, most of hIDO binds dioxygen to form the oxy species first with a rate of 5.3∗10(5) M-1s-1. It is followed by rapid binding of the substrate, L-Trp, with a rate of ca. 9.0∗10(6) M-1s-1, to form the ternary complex, L-Trp-bound oxy species of hIDO.